Electroluminescent (EL) phosphors are incorporated into thick film AC electroluminescent devices used for backlighting liquid crystal displays (LCD), for automotive dashboard and control switch illumination, and for emergency egress lighting. Some important characteristics of these EL devices include brightness, color and halflife. Such characteristics depend inherently on the phosphors which are incorporated into the EL devices.
One particularly important class of EL phosphors are the copper-activated zinc sulfide phosphors, ZnS:Cu, which have commercially desirable brightness and color characteristics. Such phosphors may be blue, green or yellow-orange (co-activated with manganese) emitting. These phosphors may also contain other activators and coactivators such as A1, C1 and Br.
U.S. Pat. No. 4,859,361 to Reilly et al. and WO 91/16722 to Faria, which are hereby incorporated herein by reference, describe generally how to make copper-activated zinc sulfide phosphors. First, the phosphor precursor materials, ZnS, a copper source and a chloride flux are mixed together and heated in a first step firing to form a hexagonal ZnS material containing copper and chlorine ions. Next, the hexagonal ZnS is subjected to low intensity milling (or mulling) to convert some of the hexagonal ZnS to its cubic crystalline form. And finally, the milled material is then blended with zinc sulfate and copper sulfate and refired at a lower temperature to form an electroluminescent phosphor.
The particle size of the EL phosphor is an important consideration in the manufacture of EL devices. For fabrication techniques such as screen printing and for EL devices requiring thinner phosphor layers, it is desirable to use phosphors having smaller particle sizes. Small size phosphors may be obtained by screening phosphors produced according to known methods in order to eliminate the coarser fraction. Unfortunately, ZnS:Cu EL phosphor having an average particle size of less than 25 micrometers have been undesirable because of inferior brightness and halflife characteristics. In addition, screening the phosphors generates considerable waste in removing the oversize fraction which can be as much as 50% of the production lot.
Thus, it would be an advantage to provide a ZnS:Cu EL phosphor having an average particle size less than 25 micrometers in diameter and having better halflife and brightness characteristics. It would also be an advantage to produce small size EL phosphors without additional processing steps to reduce the particle size.